The present invention relates to coil springs for cartridge magazines, which are more or less oval shaped with each coil consisting of curved sections at the narrow ends and straight sections on the sides. Such springs are technically a combination cantilever-coil spring.
The free-length of a compression spring is the full length to which it extends when no force is applied to it along its longitudinal axis. On the other hand, the solid-height of such a spring is its length when it is fully compressed. Since the total amount that a given spring can be compressed is the difference between its free-length and its solid-height, it is apparent that available compression depends in large part on its solid-height, which in turn usually depends on the diameter of the wire and the number of coils used. As a practical matter the potential free-length, and therefore the available compression, of a conventional spring having a given solid-height is limited within narrowly defined ranges well recognized in the art.
In cartridge magazines for firearms which employ a spring to urge the cartridges into a dispensing position within the magazine, space limitations imposed primarily by the design of the firearm, make it highly desirable to provide a spring having a greater compression to solid-height ratio than is attainable with springs of conventional design, so that the usable space within the magazine is increased because of the smaller amount of space occupied by the spring at solid-height. Furthermore, other things being equal, a spring with a longer free-length will perform better than one having a shorter free-length. However, as hereinbefore mentioned, conventional spring designs preclude increasing potential compression or free-length because such springs are designed in each particular application to provide maximum free-length for a given solid-height. It necessarily follows, therefore, that the original magazine spring for a cartridge magazine usually can not be replaced by a conventional spring for the purpose of increasing the usable space and, therefore, the capacity of the magazine.
While telescoping springs known in the art can have a greater compression to solid-height ratio as compared to non-telescoping springs, up to now they have not been satisfactory for use in cartridge magazines. Telescoping springs usually consist of two or more separate springs, one disposed within the other and produce undesired variations in spring rates. Such a spring is not self-telescoping in the sense it is a single spring having coils of different sizes which fit one within the other. On the other hand, the type of spring commonly referred to as bed springs, such as those shown in the patent to Wilson U.S. Pat. No. 1,751,261, for example, are not suitable as magazine springs because they have a very high variable spring rate and do not conform to the shape of a cartridge magazine. The same is true of barrel springs and other types of springs which are formed by sequentially arranged conical sections, such as those shown in the patent to Buckley U.S. Pat. No. 274,715.
Cartridge magazine springs, unlike most other types of springs, are generally compressed to near their solid-height and are therefore subjected to greater internal stress than springs of comparable size which are usually not compressed to solid-height during use. However, the spring action of magazine springs is provided for the most part by the bending action in the straight sections of the spring rather than by the torsion produced in twisting the curved portions. Consequently, the internal stresses in magazine springs are not as great as in typical coil springs made up entirely of round helical coils, wherein the spring action is provided entirely by torsional stresses. In order to design a self-telescoping helical coil spring, it would be necessary to provide enough space between the inner and outer coils to permit the portions of the coil which join the inner and outer portions of the coil to bend laterally as well as to twist. Because such portions are round or otherwise curved, they would be subjected to considerable added stresses and would probably be permanently distorted out of shape if used as a spring for a cartridge magazine.
It is the object of the present invention to provide a cartridge magazine spring for firearms having increased free-length, and therefore available useful compression, as compared to prior magazine springs having the same solid-height. Stated another way, the object of the invention is to provide a magazine spring having a solid-height which is less than that of prior magazine springs designed for a particular firearm, when reduction in the space occupied by the spring at solid-height can mean extra rounds in the magazine.